Effects of laser-focusing geometry on underwater laser ablation and LIBS measurement of a submerged target

Abstract

The characteristics of underwater laser-induced plasma and its corresponding LIBS signals greatly rely on the configurations of laser-focusing optics. In this work, we investigated the effects of laser focusing geometry, including the laser focusing angle, spherical aberration and lens-to-sample distance (LTSD), on the laser ablations and LIBS measurement of a solid target immersed in water. Multi-modal data of LIBS spectra, bubble images, and plasma acoustic signals were recorded using the spectroscopic method, shadowgraph method and hydrophone measurements, with varying laser-focusing geometries. It was shown that higher intensity and stability of underwater LIBS signals can be acquired using aberration-minimized focusing in combination with a larger focusing angle and a precisely chosen LTSD. Considering the much closer breakdown threshold of solid and water (compared with that of solid and air), the key factor in optimizing laser-focusing geometry is to suppress the phenomenon of multiple plasma formation caused by water breakdown, which can lead to a severe shielding effect on the laser ablations of the target and deteriorate LIBS signals significantly. Under an optimized focusing position below the target surface, higher ablation efficiency can be achieved, which corresponds to a stronger LIBS signal and a stronger acoustic signal, together with a larger bubble size and a larger ablation volume. The correlations between laser ablation characteristics provide a simple way to monitor underwater plasma formation and laser ablation conditions using acoustic signals, which can be easily measured using a hydrophone immersed in water.